Mx135 5th Gear Same Power As 4th Gear

Sir newbee here. I'm an owner of mx135 model 2015. I'm having a problem with my mc, my top speed for 4th gear is only 100kph and then when I Shifted it to 5th gear the speed is never goes up, I'm sure I shifted it to 5th gear gear because the of the sound of my engine. But I feel my 5th gear does have a power it only remain to 100kph. I hope you can nelp me in this matter.. the only modification I did is the Sprocket combination 15/42 and open pipe for my exhaust.